Level measurement using radar is based on a distance measurement in which the transit time of a radar signal or high-frequency signal to a surface of a filling material in a container is determined and the distance from the emission device of the radar device, for example an antenna horn, to the filling material is derived therefrom.
Normally in radar devices the fundamental mode of the radar signal is excited. In a circular waveguide, this is the H11 mode (also known as the TE11 mode). Once the waveguide or the pipe diameter is greater than the uniqueness range, more and more modes are able to propagate as the diameter increases. For example, with a pipe diameter of 82.5 mm and a frequency of 25 GHz, 233 modes are able to propagate.
In particular for level measurement in stand pipes or bypass lines, the influence of flange connections, pipe outlets and mixing openings, such as holes or slits, can reduce the measurement accuracy, since the different modes are reflected at different intensities at interfering locations of this type.
The different propagation speeds of the different modes can lead to dispersion; in other words, the different transit times mean that a large number of echoes of different timings are produced by the same fill level. In addition, constructive and destructive superpositions lead to amplitude fluctuations, which may also result in measurement inaccuracies.